Many prior art digital communication systems use a pulse-shaping filter, and often some form of Nyquist-type filtration, such as Nyquist, root-Nyquist, raised cosine-rolloff, etc., to shape and spread the information conveyed in each unit interval of modulated data over many unit intervals in a manner that dramatically reduces the spectrum required to transmit the information but permits efficient recovery of the information without significant intersymbol interference. Lower alpha (α) values for the pulse-shaping filter are more desirable because lower values lead to reduced spectrum requirements. Unfortunately, as α is reduced, the peak power needed to faithfully reproduce the filtered, spectrally constrained signal increases. A similar dilemma occurs in connection with applications where many channels are combined into a composite signal prior to amplification, regardless of modulation type and independent of pulse shaping. Such composite signals are commonly generated at hubs and base stations. As a result of combining multiple signals, the peak-to-average power ratio of the resulting composite signal increases.
A communication signal with a high peak-to-average power ratio is undesirable because such a signal requires the use of a high quality linear amplifier capable of amplifying the greatest signal peaks. As peak-to-average power ratio increases, power amplifier costs likewise increase to accommodate increasingly high and increasingly infrequent peaking events. Since a power amplifier often represents one of the more expensive items in a transmitter circuit, a need exists for reducing peak-to-average power ratios in communication signals so that inexpensive power amplifiers may be used and so that power amplifiers are used efficiently.
The problem of reducing peak-to-average power ratios in communication signals is difficult to solve. One technique applies hard limiting to the communication signal to prohibit the otherwise greatest peaking events from occurring in the first place. However, this is a highly undesirable solution because it leads to dramatic spectral regrowth. A moderately better, but still undesirable, technique uses a spectrally inefficient pulse shape in the pulse-shaping filter to limit the greatest signal peaks. But, this technique still suffers from an undesirable amount of spectral regrowth. Other complex techniques require such extensive processing capabilities that they are not practical in connection with high-throughput or continuous, rather than burst, transmission applications, i.e., those greater than 0.5 Mbps, such as the transmission of high-speed video data through a single channel or other data through multiple channels in parallel.
U.S. Pat. No. 6,104,761, entitled “Constrained-Envelope Digital-Communications Transmission System and Method Therefor,” by McCallister et al., is incorporated herein by reference. U.S. Pat. No. 6,104,761 teaches a technique to reduce the peak-to-average power ratio of a communication signal without enduring significant amounts of spectral regrowth. Using the technique of U.S. Pat. No. 6,104,761, the greatest peaking events are detected and compensated by adding spectrally efficient corrective pulses to the communication signal. In order to be spectrally efficient, the corrective pulses typically have a duration of several unit intervals.
While the technique of U.S. Pat. No. 6,104,761 produces adequate results for many applications, it has been discovered that the corrective pulses themselves may combine in certain situations in a manner that leads to an undesired signal peak, even though an acceptable signal level would have been present had the corrective pulses not been applied. As a result, while significant peak-to-average power reductions were observed, such peak-to-average power reductions were not as great as they could have been.